As a consequence of recent rapid development of flat panel display technologies, thin film transistors (TFTs) are being actively utilized in the implementation of two types of large area electronic devices, namely liquid crystal displays (LCDs) and flat-panel imaging devices. These devices generally comprise a large number of TFTs, which act as switches or amplifiers.
As is well known in the art, a typical TFT is constructed using a MOS structure (metal oxide semiconductor) comprising a semiconductor film, a gate electrode, a gate dielectric film, source and drain electrodes. The semiconductor film can be fabricated from amorphous silicon (a-Si), poly-silicon (poly-Si), cadmium selenide (CdSe), or other suitable semiconductor material. The metal material of the electrodes can be fabricated from chromium or aluminium. The material of the dielectric film is fabricated typically from one of either silicon nitride, silicon oxide or various anodic oxide films.
As is well known, MOS transistors are normally provided with an overlapping area between the gate and source and between the gate and drain electrodes, to ensure continuity of the channel formed in the semiconductor layer. Generally, the overlapping area should be no less than the design rule of a particular TFT device. Two parasitic capacitances (Cgs and Cgd) are formed in the overlapping areas between gate and source and between gate and drain, respectively. As a consequence of these known parasitic capacitances, gate control pulses are known to feedthrough the semiconductor layer into the source or drain, thereby deteriorating switching performance. While this is a well known common problem for all MOS transistors, the problem is exacerbated in large area TFT matrix applications where design rules must provide sufficiently large tolerances, corresponding to the lithographic tolerances of the fabrication process on a large size exposure area.
When a TFT switch turns off, the feedthrough charge comes from two components. The first is the differential component of the gate pulse on the parasitic capacitor, and the other results from channel electrons which are split away and squeezed into the source and drain electrodes (Z. S. Huang, Y. Katayama and T. Ando, "The dependence of the parasitic capacitance and the reset potential level in a solid-state imaging sensor," Proceedings of the Joint Meeting of 1989 Electric & Electronic Institutes, Tokai Shibu, Japan, P. 325, October (1989) and Z. S. Huang and T. Ando, "An analysis of reset mechanism in a stacked and amplified imaging sensor," Journal of the Institute of Television Engineers of Japan, Vol. 46, no. 5, pp. 624-631, May (1992)).
For a TFT-LCD, when the TFT turns off, negative charges are left on the pixel capacitor, causing the bias voltage of the liquid crystal to drop. This is equivalent to applying a DC voltage directly on the liquid crystal. This DC bias voltage causes the characteristics of the liquid crystal to shift in one direction, causing crosstalk. Moreover, because the capacitances of a liquid crystal in the ON and OFF states are different, feedthrough charges generate different feedthrough voltage shifts for "white" and "black" pixels. This causes image sticking and flicker noise in the TFT-LCD, a phenomenon referred to as "image persistence" in I-Wei Wu, "High-definition displays and technology trends in TFT-LCD", Journal of the SID, 2/1, pp. 1-14 (1994).
The problem of feedthrough charges in TFT LCD applications is less serious when compared to the problem of feedthrough charges in imaging sensors since the signal voltage is extremely small. Feedthrough charges in imaging applications can result in saturation of the feedback capacitor in the readout charge amplifier of a TFT matrix causing latch-up of the amplifier. One solution to this problem involves incorporating a larger feedback capacitor in the charge amplifier. However, that approach sacrifices the sensitivity of the amplifier, as discussed in I. Fujieda et al., "High sensitivity readout of 2D a-Si image sensors," Japanese Journal of Applied Physics, Vol. 32, pp. 198-204 (1993).
Furthermore, feedthrough charges in imaging applications affect not only the source or output portion of the TFT but also the drain or pixel electrode portion. In this case, excessive negative charge fed into the pixel capacitor can prevent the TFT from turning off so that charge leaks into the data line. For high-level incident light or radiation, the leakage current drops quickly, whereas for low level light or radiation, the leakage current can remain high before a subsequent charge readout. This can result in crosstalk or smearing of the image along the data line and a consequential deterioration of image quality.
Several TFT structures and specialized driving schemes have been proposed to alleviate the problem of image quality deterioration caused by charge feedthrough in TFT arrays. The most common prior art approach involves incorporating an additional storage capacitor in each pixel of the TFT array. However, that approach suffers from the disadvantage of decreasing the fill factor of the TFT imager or LCD and increasing the probability of an interlayer short circuit.
Self-alignment fabrication processes constitute another approach to reducing parasitic capacitances. Using self-aligned techniques, a channel length can be created which is almost exactly the same length as the bottom gate by using the bottom gate pattern as a photo-mask and flooding the backside of the glass substrate with ultraviolet light, as discussed in the reference of I-Wei Wu cited above. There are two known types of self-alignment TFT structures. The first is referred to as the "non-complete" self-alignment type TFT, which is capable of reducing parasitic capacitance but incapable of removing it completely. The second is referred to as the "complete" self-alignment type TFT, in which lift-off techniques are utilized. However, the lift-off techniques contribute to complexity of the fabrication process and cannot be used for a top gate TFT structure, which is the preferred structure for many imaging sensors such as the amorphous selenium/cadmium selenide TFT SAMURAI radiation imaging sensor (W. Zhao and J. A. Rowlands "A large area solid-state detector for radiology using amorphous selenium," SPIE Vol. 1651, Medical Imaging VI: Instrumentation, pp. 134-143, (1992)).
In terms of prior art driving schemes, four kinds of LCD driving methods (Frame inversion, Gate line inversion, Data line inversion and Dot inversion method) have been proposed (Reference: I-Wei Wu, "High-definition displays and technology trends in TFT-LCDs," Journal of the SID, vol. 2, no. 1, pp. 1-14, 1994). These driving methods change the polarity of bias voltage on the liquid crystal film periodically, and they reduce the sticking and crosstalk effects by averaging noise in the time or spatial domains. However, none of these is capable of driving an imaging sensor, because changing polarity of bias voltage on a detector film is usually not allowed. Most of these prior art photodetectors do not show symmetric characteristics as altering the polarity of bias voltage. Furthermore, it is difficult to change the polarity of bias voltage in some X-ray detectors such as a-Se or a-Si x-ray detectors, where the bias voltage can be tens of KV.
Another prior art approach to solving the problem of parasitic capacitances is the use of a dual gate MOSFET comprising two series connected FETs fabricated on a silicon substrate (N. Ditrick, M. M. Mitchell and R. Dawson, "A low power MOS tetrode", Proceedings of International Electron Device Meeting, 1965). This device is known to be characterized by relatively low switching noise. When the dual gate FET is used as a switch, one gate is grounded and the other gate is connected to a gate control pulse for turning on and turning off the switch. Since the feedthrough charge is shunted to ground for the control gate, the capacitance Cgd equals zero. However, this dual gate structure cannot be applied to TFT-LCDs or TFT image sensors without decreasing the fill-factor.
Neither the self-alignment type TFT nor the dual gate TFT discussed above, eliminate feedthrough resulting from split channel electrons.
In addition to the problems mentioned above, the split channel electrons also cause partition noise in the imaging sensors, as described by N. Teranishi et al in the paper: "Partition noise in CCD signal detection," IEEE Trans. on Electron Devices, vol. 33, no. 11, pp. 1696-1701. The partition noise is proportional to the channel area, in which electrons are pumped in and pumped out.